Conventional SF.sub.6 gas bushings used in high voltage circuit breakers have no significant shielding to alter the distribution of electrical potential lines. They, therefore, have very asymmetrical potential distributions, with most of the external electrical stress located at the base of the bushing.
FIG. 1 shows a longitudinal cross section of a conventional SF.sub.6 gas bushing. A high voltage conductor 10 runs through the center of a hollow bushing insulator 12 that forms a housing around the high voltage conductor 10. The bushing insulator 12 is typically formed with weather sheds 13 on the outer surface of the bushing insulator 12. Weather sheds 13 are described in more detail below. A ground potential grading shield 14 (hereinafter "ground shield") is often mounted at the base of the bushing at flange 16. Flange 16 is used to connect the bushing to ground through a tank assembly (not shown) of the circuit breaker.
FIG. 2 shows an electric field plot of a conventional gas insulated bushing. As shown in FIG. 2, conventional bushings have a very asymmetrical electrical potential distribution along the outside surface of bushing insulator 12 (i.e., non-parallel potential lines) and high electrical stress throughout the bushing as indicated by the closely spaced potential lines. The voltage at which a flashover occurs is related to the bushing's voltage withstand capability. The highest electrical stress is produced at the base section of the bushing as shown in FIG. 2. The higher the electrical stress, the lower the voltage withstand capability of the bushing. Thus, the ground shield 14 provides the bushing only minimal protection from electrical breakdown in the portion of the bushing subject to the highest electrical stress. Failure of conventional bushings to shield other portions of the bushing insulator subject these insulators to high electrical stress which reduces the bushing's voltage withstand capability.
SF.sub.6 breaker bushings are an integral part of the breaker, both electrically and mechanically. They are not designed or used as general purpose apparatus bushings. SF.sub.6 breaker bushings are designed to support and insulate high voltage line connections and carry power into the grounded tank of the circuit breaker.
In high voltage circuit breakers, the pairs of bushings for each phase of the circuit protected by the breaker are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings. One means for achieving the desired spacing has been to use conical bushings such that the terminal ends of the bushings have smaller diameters than their respective bases. For example, FIG. 3 shows a high voltage circuit breaker with conical bushings 20a-c and 22a-c. The conical bushings are angled away from each other to provide an adequate air gap (AG) between their ends so that in the event of a flashover or significant current leakage, the resulting breakdown is grounded in one of the dead tanks 23 of the dead tank assembly 24. As circuit breakers become more compact, the size and spacing of the bushings become a critical design feature of the circuit breaker.
The weather sheds 13 (FIG. 1) on the external surface of the bushing insulator 12 resist the effects of rain and surface dirt to maintain good dielectric conditions and thereby reduce the potential of a flashover or leakage. Bushing insulators including the weather sheds have been made from porcelain or a cast epoxy. Typically, these weather sheds are designed so that water rolls off the sheds keeping the underside of the sheds substantially dry. However, a significant portion of the insulator surface can become wet or degraded by environmental pollution. The resulting weakening of the dielectric field can cause leakage and flashover conditions.
An additional drawback of porcelain or cast epoxy bushings is that they are relatively brittle and, therefore, are subject to damage from external conditions that can cause them to shatter so that the SF.sub.6 contained therein explodes. To provide an optimal insulator and a safe and reliable housing for the bushing conductor, the porcelain and cast epoxy insulators are produced with a relatively thick wall (i.e., about 1 inch). The increased thickness further narrows the air gap, increases the weight of the bushings, and increases the cost of the bushings.
Therefore, a composite bushing has been developed that provides the following advantages over traditional bushings: non-brittle behavior, reduced weight and wall thickness, pollution resistance, and improved wet electrical capability. A longitudinal cross section of a composite bushing is shown in FIG. 4. Composite bushings insulators are made up of a fiberglass reinforced tube 30 protected by a silicone rubber housing 32. These bushings have a straight cylindrical composite tube with aluminum end flanges 40 and 42 and room temperature, vulcanized (RTV) silicone rubber weather sheds 33. The RTV silicone rubber has a hydrophobic surface due to oil films that naturally form on the rubber surface.
A grading shield 36 is provided at the base of the bushing mounted to the flange 40 and grades the high electrical potential stress in that region as explained above. A second top end grading shield 38 is also typically required in cylindrical composite bushings to drive the voltage down from the top section of the housing to reduce the risk of breakdown between two bushings of the circuit breaker. The addition of a second shield increases the cost and weight of the bushing and adds further steps to the assembly of the bushing.
The composite bushings are produced by using an injection molding technique in which a single mold forms a single section of the housing 32 at a time. This process is both time consuming and relatively inefficient in that each section of the housing must also be molded together to form the completed insulative housing. Since the insulative housing is formed from an injection molding process, a specially designed mold would be required to produce the desired conical shape. For many high voltage breakers that require very large bushings, such molds are impractical.
A process for molding rubber using a traveling mold has been developed. Essentially, the traveling mold is capable of forming plastic or rubber on substantially any shape in a continuous process. Therefore, to improve the performance and reduce the size, weight and number of parts of high voltage bushings there is a need to design a conical composite bushing that has an insulative housing which can be formed using such a traveling mold. In addition, there is a need to grade the high electrical stress over a greater portion of the bushing.
Floating shields and foils, have been employed for the purpose of dividing the voltage into sections, resulting in a forced, more symmetrical electrical potential distribution on the outside of the insulator. This more symmetrical, potential distribution (i.e., parallel graded potential lines) results in lower electrical stress and, therefore, a higher voltage withstand capability. The floating shields also result in a more symmetrical potential distribution inside the insulative housing, which results in lower electrical stress. This lower internal stress can allow smaller diameter bushings for the same voltage withstand level, when compared to conventional bushings.
One example of a conventional bushing utilizing a floating shield is shown in FIG. 5. As shown in the figure, an epoxy insulator 50 is provided between the high voltage conductor 52 and the interior wall of the hollow housing 54. The epoxy insulator 50 is used to support a floating shield 56 disposed along the high voltage conductor 52 and between the conductor 52 and the epoxy insulator 50. The epoxy insulator 50 comprises a number of sections that are connected together at joints 58 and 59. It should be understood from the potential lines shown in FIG. 2, that the epoxy insulator 50 is placed in a very highly stressed area of the bushing. Thus, the joints 58 and 59 at which the sections of the epoxy insulator are joined must be nearly perfect to avoid electrical failure of the bushing. Moreover, the epoxy insulator requires mounting at both the top end 57 and at its base 60. The epoxy insulator shown in FIG. 5 is critically stressed in high voltage applications and may, therefore, be subject to reliability problems. Therefore, although this floating shield is capable of grading a greater portion of the bushing than the ground shield shown in FIG. 1, its assembly requires precision and several additional parts thereby increasing its cost and reducing its reliability.
Foil shield designs typically employ solid "cores" of layered epoxy and foils, or foil/paper/oil cores. These designs can effectively force symmetrical field distributions to grade the potential lines in bushings radially, but also force high stresses onto solid insulation. In particular, the solid insulation (paper impregnated with oil) is located between the foils of non-magnetic metals in highly stressed areas. If any airpockets or other debris are left in the layered foils, the performance of the bushing can be critically degraded. Accordingly, these foil shields require extensive precision and are also costly to produce.
Therefore, there is a need to provide a high performance shielded bushing having an optimal voltage withstand capability achieved by optimizing the weather and environmental tolerance of the bushing and reducing the electrical stress on the bushing's insulative housing while minimizing the size, weight, cost, and complexity of the bushing.